1. Field of the Invention
The invention relates to methods and apparatus for forming tubular products and, more particularly, relates to methods and apparatus for forming tubular products such as filters from nonwoven polymer fibers, and also relates to assemblies for withdrawing such products from the mandrels on which the products are formed.
2. Discussion of the Related Art
The production of tubular products such as filters from nonwoven fibers is becoming increasingly popular. Such products are typically manufactured by discharging nonwoven polymeric fibers such as polypropylene from a die head or spinnerette and depositing the fibers onto a rotating mandrel. The fibers are intertwined and wrapped around the surface of the mandrel as they are deposited onto the mandrel, thus forming a tube. The tube is pulled axially along the mandrel as the fibers are deposited, thus producing a layered tubular element the thickness of which and porosity of which is determined by, among other things, the rotational velocity of the mandrel and the speed at which the tubular element is moved axially along the mandrel.
The production of a tubular element of the type described above requires that the element be withdrawn from the distal end of the mandrel during the manufacturing process, thus producing a continuous tube which is sawed or cut in some other manner to form the final tubular product. Several devices for withdrawing a tube from a mandrel are disclosed in U.S. Pat. No. 3,933,557, which issued to Pall on Jan. 20, 1976 (Pall '557), U.S. Pat. No. 4,116,738, which issued to Pall on Sep. 26, 1978 (Pall '738), and U.S. Pat. No. 4,847,125, which issued to Schwarz on Jul. 11, 1989 (Schwarz). Each of these patents discloses a process whereby a tube formed from nonwoven fibers is withdrawn from the mandrel via contact with a rotating element located at or downstream from the mandrel. Pall '557 and Pall '738 use canted rollers to withdraw the tube, and Schwarz uses gears of an unspecified construction. While each of these devices adequately withdraws the tube from the mandrel, each exhibits several drawbacks and disadvantages.
For instance, these systems are relatively bulky and complex. Moreover, because of slippage between the roller or other element and the tube, it is difficult to vary or even to control the speed at which the tube is withdrawn from the mandrel. Such control can be important because the rate of withdrawal can affect the properties of the tube being produced. Controlling the rate of withdrawal thus controls the properties of the tubes.
Moreover, none of these systems is capable of heat treating the tube in an in-line process.